Means for generating motive force, such as electric motors, internal combustion engines, and the like, generate power most efficiently when they operate within a relatively narrow rpm range. For example, an internal combustion engine which is designed to operate within a relatively narrow range of rpm's operates about 20-30% more efficiently than an internal combustion engine having the same power capacity which operates over a wide range of rpm's. With apparatus employing electrical motors such as power tools and the like, variation of rotational speed (rpm) is generally achieved by pulsing the input power. Pulsing the input power, while changing the rotational speed of the electric motor, also reduces its total power. Other methods of changing the speed of electric motors, reduce the output power of the electric motor to achieve variable rotational speeds. With apparatus employing internal combustion engines or the like, a transmission mechanism is generally provided linking output from the engine with the drive means, for providing different gear or drive ratios between the engine and drive wheels, for example, of a motor vehicle. The principal function of the transmission is to provide acceleration from rest to a high speed through a wide speed range, while permitting the engine to operate efficiently within narrow rpm ranges.
Two types of transmissions are typically employed in motor vehicle applications. Manual transmissions require a clutch mechanism to disengage the engine from the transmission while gears in the transmission are displaced manually to provide different drive ratios between the engine and the drive wheels. Conventional motor vehicle automatic transmissions employ frictional mismatch devices, such as a hydrodynamic torque converter and spring energized clutches, to provide a cushion between mismatched transmission ratios. Both of these types of conventional motor vehicle transmissions vary the transmission ratios in predetermined, discrete steps. This requires the engine to vary RPM within the domains of these steps to compensate for mismatch between steps, resulting in inefficient conversion of motive force to driving force.
Continuously variable transmissions (CVT's) which provide a continuously variable transmission ratio range have also been developed for motorized vehicle applications. Prior art continuously variable transmissions provide efficient conversion of motive force to drive ratios but they require an automatic or manual clutch for disengaging the engine during stopping and reverse, and they utilize frictional devices which reduce their overall efficiency. CVT's employing a continuous rubber belt supported between two variable diameter pulleys have been used in lightweight vehicles such as snowmobiles and mopeds. CVT's having a segmented metalized belt supported between two variable diameter pulleys have been developed for commercial use and tested in small automotive engines, but they are limited in their applications, since reasonable horsepower capacity to size ratios have not been achieved. One type of CVT employing a continuous segmented metalized belt supported between two variable diameter pulleys is described in "Finally--CVT," Popular Science, September, 1987, p. 56. U.S. Pat. No. 4,497,221 teaches a similar type of apparatus employing a fixed walking gear and a variable pitch gear chain constrained by a mechanism which expands and contracts the diameter of the gear chain.
Controllers utilizing various types of differential gear assemblies have been suggested as low-cost, rugged alternatives to electric transmissions. Differential gear assemblies, in general, transmit power with variable ratios of input to output speed, while maintaining substantially steady torque at all speeds. A split belt and chain CVT, for example, may be used as a control feature for this type of differential transmission, as disclosed in "Differential Gearings--Controlling High-Power Transmissions," Machine Design, Apr. 21, 1988, p. 131. Although this method of differential output modification has many advantages, its performance is substantially limited by the limited torque capacities and ratio ranges achieved by existing types of CVTs.
These torque limitations on prior art CVT TECHNOLOGY are insurmountable. For example, CVTs that rely on surface area of contact transfer torque by traction between belts and pulleys. The torque transferred is dependent on surface area of contact. CVT's change ratios by reducing effect of diameter of one pulley while increasing the effect of diameter of the other pulley. Reducing effective diameter reduces the surface area of contact. Therefore, variable diameter pulley means for controlling ratios reduce power capacity proportionate to the ratio ranges controlled. On the other hand, with regard to utilization of belts, the relationship of belt "surface area of contact" to belt diameter, pulley diameter, pulley center-to-center distance and belt strength are interrelated and optimized for the best power for size and weight. To double the torque capacity of a CVT the surface area of contact must be doubled. Doubling the surface area of contact increases all other aspects proportionately. Therefore, in order for a CVT used with a 60 horsepower engine running at 4,200 RPM to develop 75 foot pounds of torque, it would require a 12" distance from center-of-pulley to center-of-pulley and 18" overall in height from edge-of-pulley to edge-of-pulley. To make a comparable CVT for handling 1,200 foot pounds of torque generated by a 411 horsepower engine running at 1,800 RPM (Diesel Truck), it would require a distance of 4' center-of-pulley to center-of-pulley and distance of 6' from edge-to-edge with 4' diameter pulleys.
Efficient power transfer from the power generator to the drive means is, of course, a primary objective of transmission ration changing devices. Mechanical devices generally demonstrate some power loss due to friction. Devices which generate more friction demonstrate proportionally higher power losses. Mechanical devices utilizing bands, belts, and the like, rely on friction to provide the driving traction, and the friction generated by the relative motion between component belts and their driving means dissipates some of the usable power as heat. Gear drives which do not utilize bands, belts or other frictional devices, generally create substantially less friction, and consequently provide more efficient power transfer. In addition, gear drives generally have large ratios of torgue capacity to size and weight, and thus provide an efficient transfer of rotary motion without contributing significantly to the load.
Variable speed gear drive mechanisms which generate and utilize epicyclic motion are known in the art. For example, U.S. Pat. No. 1,660,356 teaches an epicyclic speed change gear wherein epicyclic motion is generated and modified through the use of gear sets to produce a direct output. U.S. Pat. No. 3,955,435 and 3,087,355 similarly teach means for generating epicyclic motion and processing the epicyclic motion to produce a direct output. According to the teachings of each of these patents, sinusoidal, reciprocating rotational motion is collected serially from each of several generators and conveyed to an output shaft. Since each epicyclic generator is out of phase with respect to the other epicyclic generators, a pulsating output is produced at the output shaft which represents the peaks collected from each of the epicyclic generators and arranged serially. The output from these types of devices, while attempting to approximate a uniform rotational output is quite irregular in fact. Irregular output is undesirable for a variety of reasons, primarily because it places large periodic stresses on the component parts which result in damage to the component parts and reduce the operating lifetime and reliability of the device.
Serial processing of epicyclic motion in this fashion has several serious drawbacks. Output generated in this fashion, although it may simulate a uniform output, is not uniform, since it represents only serial collection of peaks from a number of epicyclic motion generators. The amplitude of this output is proportional to the amplitude of epicyclic motion generated, which is proportional to the degree of eccentricity of the epicyclic device. Because each epicyclic generator contributes serially to the output, at any point in time, a single epicyclic generator is contributing the full output. Consequently, the torque capacity of the device is limited to the torque capacity of each single epicyclic generator. To provide sufficient torque capacity, relatively large and heavy mechanical devices must be employed, which reduces the overall efficiency of the system. In many transmission ratio changing applications, such as motor vehicle applications, it is important for all components to have ratios of high strength to size and weight. This is particularly important in automotive applications wherein the objective is to achieve greater fuel efficiency with smaller and lighter vehicles. It is important for many applications that the power transmission mechanism represent as small a percentage of the total payload as is reasonably practical.
An important objective of transmission ratio changing devices is to provide proper adjustment of the torque/rpm ratio to achieve the desired output speed. In other words, at each output speed, the full torque should be delivered as output for the power generated. Only under these conditions will efficient power transfer be achieved. Proper balancing of a mechanical transmission ratio changing device is also important to reduce frictional losses, and to prevent vibration of the device or its components. Vibration causes stress on component parts which may significantly reduce their operating lifetime and reliability.